Transmissions with hydraulically actuated clutches (e.g., continuously variable transmissions (CVTs) and power shift transmissions (PSTs)) are well known in the art. Typically, a hydraulically actuated clutch includes a cylindrical enclosure or can containing clutch plates configured to be engaged/disengaged with one another so as to engage/disengage the clutch. Engagement of the clutch plates is achieved by actuating a hydraulic piston relative to one or more of the plates. Specifically, pressurized hydraulic fluid is supplied to a chamber associated with the piston such that the pressurized fluid actuates the piston in a manner that engages the clutch plates. When the pressurized fluid is no longer being supplied to the chamber, a clutch or return spring may be configured to apply a biasing force that pulls the clutch plates apart and returns the piston to its unactuated position, thereby disengaging the clutch.
For many hydraulically actuated clutches (e.g., large wet clutches), it is often difficult to prevent self-energizing of the clutch when it is spinning at a relatively high speed. Specifically, if there is fluid behind the piston and the clutch can is spinning, the fluid exerts a pressure on the piston due to the centrifugal effect. In certain instances, the force applied by the fluid may be sufficient to overcome the biasing force applied by the return spring, thereby forcing the clutch into engagement and resulting in a centrifugal clutch lock-up. Such a lock-up can result in the clutch being significantly damaged or even destroyed.
To prevent centrifugal clutch lock-ups from occurring, clutches are often provided with a pressure relief valve that is configured to allow fluid to escape from the piston chamber, thereby reducing the fluid pressure within the chamber. Unfortunately, instances occur when the valve does not properly open after a clutch has been turned off. In such instances, a centrifugal clutch lock-up may still occur if the speed of the associated clutch can reaches a given lock-up speed.
Accordingly, an improved system and method for preventing centrifugal clutch lock-ups, particularly when an associated pressure relief valve does not open when a clutch is commanded to be disengaged, would be welcomed in the technology.